Optical systems using holography have a wide scope of application, among which is the measurement of the deformation of an object by employing such techniques as double-exposure holography or real-time holography. For this purpose, a hologram is recorded by directing an object beam (typically a laser beam reflected from the object to be measured) and a reference beam to strike a recording material at different angles. An interference pattern between the two beams is formed as a hologram on a recording material. The recording material is typically made of a substance such as a single crystal of BSO or BGO which is capable of real-time recording. The recording material, when subsequently illuminated by a coherent beam that is introduced in the same direction as the original reference beam, reconstructs the image of the object. This recording method enables the technician to make an extremely precise measurement of the slightest deformation occurring in an object.
The holographic apparatus typically includes half mirrors for splitting a laser light into two beams and mirrors for reflecting and guiding the beams. Since the interference pattern is formed by the difference in the length of the path of each optical axis, it is necessary to strictly control this length as well as the setup angle of each mirror. Thus, a dislocation or vibration in the order of a wavelength of light would render the recording of the desired hologram impossible.
In order to meet these requirements, the holographic apparatus is typically set up on a large heavy table which is made of an iron block measuring 1500 mm long, 700 mm wide and several millimeters thick. The laser is directly placed on the iron table, and the lenses, mirrors and recording material are fixed on the table by respective magnet stands. The iron table is elastically supported by tires or springs which insulate the table against the effects of external vibration and impact. In some models, instead of using one magnet stand for each component, the lenses and prisms are put in an enclosure which is fixed onto the iron table. In either case, the entire holographic apparatus has to be installed on the large heavy table which typically weighs as must as 100 kg. The massiveness of the holographic apparatus is the primary reason why the system is still utilized only at a laboratory level. In addition, this holographic apparatus is far from being a consistent and reliable optical system; specifically, since the spatial arrangement of discrete optical components is vulnerable to vibrations or other accidental dislocation, the holographic apparatus would be virtually inoperable outside a highly controlled laboratory environment. These problems are common to other image processing systems such as those involving Fourier transform and video correlation.